Dear Lauren

The answer to your question is to be found in the Judiciary Act 1903 (Cth).

Section 15 of the Judiciary Act provides:

The jurisdiction of the High Court may, subject to the provisions of this Act, be exercised by any one or more Justices sitting in open Court.

Section 19 of the Judiciary Act provides:

Except as hereinafter provided, a Full Court may be constituted by any two or more Justices of the High Court sitting together.

Section 20 of the Judiciary Act provides that, amongst other things, when the jurisdiction of the High Court is engaged to hear and determine appeals from judgments of:

(a) of a Justice of the High Court exercising the original jurisdiction of the High Court; or

(b) of the Supreme Court of a State exercising federal jurisdiction when such jurisdiction is exercised by a single Judge; or

(c) of any other court exercising federal jurisdiction,

then the High Court's jurisdiction must be exercised by a Full Court.

Since a Full Court can be constituted by 2 or more Justice sitting together, many cases are heard by 5 Justice sitting together.

Given that the jurisdiction of the High Court need not be exercised en banc, there is no need to have seven Justice sit together to consider every single matter. Seven Justices tend to sit in matters of considerable Constitutional importance.

Incidentally, the jurisdiction of the High Court to hear and determine appeals from judgments of the Supreme Court of a State sitting as a Full Court (or as the Court of Appeal of the relevant State), shall be exercised by a Full Court consisting of not less than three Justices: Judiciary Act, s 21.

I hope this assists.